Spectroscopy in Structure Determination Questions and Answers

Answer: A

Fluoroacetone (FCH₂COCH₃) shows two doublets.

It has two kinds of protons. The CH₂F protons are split by the fluorine (¹⁹F, I = 1/2) on the same carbon, a large geminal coupling of about 47 Hz, giving a doublet. The CH₃ protons are four bonds from the fluorine, through the carbonyl group, and show a small long-range H-F coupling of a few hertz, so they too appear as a doublet.

The two sets of protons are separated by the carbonyl carbon and do not couple with each other, so no triplet arises. Treating the methyl group as a singlet ignores the four-bond H-F coupling that the key counts.

Answer: D

The order is I > III > II: vinyl acetate > methyl acrylate > butanone.

A ketone such as butanone (II) absorbs near 1715 cm^-1. In an ester the alkoxy oxygen withdraws electrons inductively, which strengthens the C=O bond, so saturated esters absorb near 1740 cm^-1.

Vinyl acetate (I) is an enol ester: the oxygen's lone pair is shared with the C=C bond, so it donates less to the carbonyl, and the C=O frequency rises to about 1770 cm^-1. Methyl acrylate (III) is an alpha,beta-unsaturated ester: conjugation of C=C with C=O lowers its frequency to about 1725 cm^-1, still above the ketone. So I > III > II, and the orders placing the ketone first or the acrylate above the enol ester are wrong.

Answer: C

The spectrum shows two doublets and a singlet.

At -98°C rotation about the C-C bond is slow on the NMR time scale, so each staggered rotamer gives its own spectrum. Looking along the C-C bond, the CF₂Br carbon carries F, F and Br, and the CBr₂CN carbon carries Br, Br and CN.

In the rotamer where the Br of CF₂Br is anti to the CN group, a mirror plane makes the two fluorines equivalent, giving a singlet. In the other two rotamers that Br is anti to one of the bromines; these two are mirror images with identical spectra, and in them the two fluorines are non-equivalent and couple with each other, giving an AB pattern of two doublets. Two singlets or two doublets alone would leave out one kind of rotamer.